Molecular orbital breaking in photo-mediated organosilicon Schiff base ferroelectric crystals

Ferroelectric materials, whose electrical polarization can be switched under external stimuli, have been widely used in sensors, data storage, and energy conversion. Molecular orbital breaking can result in switchable structural and physical bistability in ferroelectric materials as traditional spatial symmetry breaking does. Differently, molecular orbital breaking interprets the phase transition mechanism from the perspective of electronics and sheds new light on manipulating the physical properties of ferroelectrics. Here, we synthesize a pair of organosilicon Schiff base ferroelectric crystals, (R)- and (S)-N-(3,5-di-tert-butylbenzylidene)-1-((triphenylsilyl)oxy)ethanamine, which show optically controlled phase transition accompanying the molecular orbital breaking. The molecular orbital breaking is manifested as the breaking and reformation of covalent bonds during the phase transition process, that is, the conversion between C = N and C–O in the enol form and C–N and C = O in the keto form. This process brings about photo-mediated bistability with multiple physical channels such as dielectric, second-harmonic generation, and ferroelectric polarization. This work further explores this newly developed mechanism of ferroelectric phase transition and highlights the significance of photo-mediated ferroelectric materials for photo-controlled smart devices and bio-sensors.


Significance:
Ferroelectric materials are widely used in various applications such as data storage, sensors, and transducers because of their robust spontaneous electrical polarization 1 .They generally undergo phase transitions which can be described as spatial symmetry breaking by Landau's phenomenological theory 2,3 .Phase transition is essential for ferroelectrics since it can bring about tunable physical properties 4 .The phase transition mechanism of ferroelectrics mainly includes two types: order-disorder and displacive type.The former is usually found in some molecular ferroelectrics, while the latter is common in inorganic ceramics such as BaTiO3 and Pb(Ti, Zr)O3 5 .These processes are accompanied by changes in symmetric elements, which is well-known as symmetry breaking.Symmetry breaking will contribute to the rearrangement of ferroic orders and changes in spontaneous polarization, thereby achieving regulation of ferroelectric properties 6 .
Recently, a new type of ferroelectric material, photo-mediated ferroelectric whose spontaneous polarization can be switched reversibly with a photoinduced phase transition triggered by structural photoisomerization has been proposed.This is a breakthrough in next-generation ferroelectric materials because photoirradiation stands out as a nondestructive, noncontact, and remote-control mean beyond an electric or strain field 7 .Among them, the typical photochromic compounds, such as Schiff base, azobenzene, diarylethene derivative, and spiropyran, have been chosen for light-driven ferroelectric materials, whose structural change is generally caused by photoisomerization instead of the traditional thermodynamic structural phase transition 8,9 .However, the conventional phase transition mechanism cannot be used to describe them.Thus, a new ferroelectric phase transition driven by switchable covalent bonds is proposed.Molecular orbital breaking can help us to understand this unconventional ferroelectric mechanism from the perspective of electronics, and further provides a new approach to modulating ferroelectric properties at the electronic level especially the polarization, which is a beneficial supplement to symmetry breaking.The core content of molecular orbital breaking is the valence bond recombination, for which the phase transition process involves the breaking and reformation of covalent bonds and thus leads to polarization changes.For example, Liao et al. disclosed the dual breaking of molecular orbitals and spatial symmetry in a photochromic diarylethene ferroelectric material 10 .This is the first time that this new ferroelectric mechanism-molecular orbital breaking is proposed.Meanwhile, such a photoinduced phase transition is entirely driven by switchable covalent bonds with breaking and reformation, enabling the reversible light-controllable ferroelectric polarization switching, dielectric, and nonlinear optical bistability.It should be highlighted that in comparison with traditional ones driven by thermal stimuli, photo-mediated ferroelectric materials will have promising application prospects in biomedicine since their physical properties can be mediated by light at a temperature matching the human body, making them promising candidates for applications in personal health management, sports detection, electronic skin, and other fields 11 .
Here, we synthesized a pair of organosilicon Schiff base ferroelectric crystals, (R)-and (S)-N-(3,5-di-tert-butylbenzylidene)-1-((triphenylsilyl)oxy)ethanamine.They show optically controlled phase transition accompanying the molecular orbital breaking.The molecular orbital breaking is manifested as the breaking and reformation of covalent bonds during the phase transition process, that is, the conversion between C=N and C-O in the enol form and C-N and C=O in the keto form.As expected, these organosilicon Schiff base compounds show good biocompatibility and promotion of cell proliferation in biological cells confirmed by the tests of cell viability and adhesion.These intriguing features enable the multi-channel modulation in physical properties without the changing of temperature, thus making Rand S-SEA-Si a beneficial supplement to traditional ferroelectrics whose switchable properties are generally realized by thermodynamics.Therefore, this finding extends the ferroelectric mechanism for the optically-controlled ferroelectrics and highlights the important and unique applications of this type of materials in biomedicine.This will attract the attention of scientists from multiple disciplines such as chemistry, physics, electronics, biomedicine, and materials science.
The discovery of a pair of organosilicon Schiff base ferroelectric crystals having photo-mediated bistability with multiple physical channels such as dielectric, second-harmonic generation, and ferroelectric polarization.

2.
We first used molecular orbital breaking to interpret the phase transition mechanism from the perspective of electronics in the Schiff base ferroelectric system.

3.
Photo-mediated organosilicon Schiff base ferroelectrics with good biocompatibility are essential for bioelectronics because they can realize the modulation of physical properties in a moderate environment compared to conventional thermodynamics ferroelectrics.